Are they in a union? I know most of the industry is unionized but I've never heard of vfx having its own or being part of one of the others.
However some of the larger vfx houses like ILM and some of the feature animation houses like Dreamworks are are union. (ILM is IATSE...not sure about others)
In my experience, whether or a vfx shop is union or not has little impact on salary and job stability.
Without unionization across the industry, that makes sense.